The study of gene function in complex genetic environments such as eucaryotic cells would greatly profit from systems that would allow stringent control of the expression of individual genes. Ideally, such systems would not only mediate an "on/off" status for gene expression but would also permit limited expression at a defined level.
Attempts to control gene activity by various inducible eucaryotic promoters responsive to, for example, heavy metal ions (Mayo et al., Cell 29: 99-108 (1982); Brinster et al., Nature (London) 296: 39-42 (1982); Searle et al., Nouer, L., CRC Boca Raton, Fla. (1991), pp. 167-220), or hormones (Lee et al., Nature (London) 294: 228-232 (1981); Hynes et al., Proc. Natl. Acad. Sci. USA 78: 2038-2042 (1981); Klock et al., Nature (London) 329: 734-736 (1987); Israel & Kaufman, Nucleic Acids Res. 17: 2589-2604 (1989)) have generally suffered from leakiness of the inactive state (e.g., the metallothionein promoter (Mayo et al., Cell 29: 99-108 (1982)) or from pleiotropic effects caused by the inducing principles themselves, such as elevated temperature or glucocorticoid hormone action (Lee et al., Proc. Natl. Acad. Sci, USA 85: 1204-1208 (1988)).
In search of regulatory systems that do not rely on endogenous control elements, several groups have demonstrated that the lac repressor/operator inducer system of Escherichia coli functions in eucaryotic cells. Three approaches have been described: (i) prevention of transcription initiation by properly placed lac operators at promoter sites (Hu & Davidson, Cell 48: 555-566 (1987); Brown et al., Cell 49: 603-612 (1987); Figge et al., Cell 52: 713-722 (1988); Fuerst et al., Proc. Natl. Acad. Sci. USA 86: 2549-2553 (1989); Deutschle et al., Proc. Natl. Acad. Sci. USA 86: 5400-5405 (1989)), (ii) blockage of transcribing RNA polymerase II during elongation by a lac repressor/operator complex (lac R/O; Deutschle et al., Science 248: 480-483 (1990)), and (iii) activation of a promoter responsive to a fusion between lacR and the activating domain of virion protein 16 (VP16) of herpes simplex virus (HSV) (Labow et al., Mol. Cell. Biol. 10: 3343-3356 (1990); Baim et al., Proc. Natl. Acad. Sci. USA 88: 5072-5076 (1991)).
At present, however, the utility of the lacR/O-based systems in eucaryotic cells is limited since the inducer isopropyl..beta.-D-thiogalactopyranoside (IPTG), despite its rapid uptake and intracellular stability (Wyborski & Short, Nucleic Acids Res. 19: 4647-4653), acts rather slowly and inefficiently, resulting in only moderate induction. Nevertheless, an interesting conditional mutant of a lacR-VP16 fusion has been described (Baim et al., Proc. Natl. Acad. Sci. USA 88: 5072-5076 (1991)). It activates a minimal promoter .about.1000-fold at elevated temperatures in the presence of IPTG. The temperature dependence and the inherent IPTG-related problems, however, may also limit this approach.